The primary circuit of a pressurized-water nuclear reactor during the course of operation contains water containing boric acid at a pressure of about 155 bars, which serves both for cooling the core of the reactor arranged in the vessel and for the control of the reactivity of this core. The primary circuit of the reactor includes a number of loops, generally three or four, in each of which is placed a steam generator which enables evaporation of the feed water by employing the heat from the reactor core conveyed by the water under pressure in the primary circuit.
Each of the loops of the primary circuit includes a portion of pipework called the "cold arm" in communication with the interior of the reactor vessel and bringing the primary water inside this vessel where it comes into contact with the core of the reactor, as well as a portion of pipework called the "hot arm", likewise in communication with the interior of the reactor vessel, through which the water heated by contact with the core is discharged towards the steam generators.
If the primary circuit exhibits a leak it is necessary to compensate it by an injection of additional water into this circuit. In the event of a very large leak and, for example, in the event of a real rupture of a pipe in the primary circuit, it is necessary to pass very large amounts of cooling liquid, consisting of water with the addition of boric acid, within a very short time in order to avoid a very high rise in temperature in the core which might cause it to melt.
In the case of large leaks it is necessary to put into operation a reactor emergency cooling device by injection of cooling liquid, called a safety injection system.
Such a device for safety injection includes at least two injection trains, each consisting of a reserve of water with the addition of boric acid, and at least one set of means for pumping at one or more different pressures, arranged outside the safety enclosure of the reactor. This device for safety injection likewise includes accumulators arranged inside the safety enclosure of the reactor, containing cooling water under pressure and capable of injecting a first amount of water very rapidly into the cold arms of the primary circuit of the reactor.
Each of the injection trains of the safety device includes as many injection pipes receiving the water from a pumping means as there are pumping means at different pressures in the train in question. Thus each of the trains may include, for example, both low-pressure, medium-pressure and high-pressure pumps. In this case the injection train will include three injection pipes, called main pipes, which pass through the safety enclosure of the reactor and are each connected inside the safety enclosure to a distributor drum enabling the injected water to be distributed into the various cold arms of the primary circuit. In each of the main injection pipes there is arranged a block valve outside the safety enclosure of the reactor.
At the outlet from the drums, the distribution of the injected water into the various cold arms of the primary circuit is effected through distributor pipes in which are arranged non-return valves enabling the return of water from the primary circuit towards the drums to be avoided.
In the case of injection trains including low, medium and high-pressure pumps, there are arranged inside the enclosure three drums which receive and distribute the water from the main pipes of each of the trains connected respectively to the low, medium and high-pressure pumping means.
In the event of a rupture of a cold arm of the primary circuit, the concentration of boric acid in the cooling liquid lying in the vessel increases continuously during the first phase of injection into the cold arms. In fact, during this injection the steam produced during the cooling of the core is released into the safety enclosure of the reactor. This steam carries along a small amount of boric acid, with the result that the concentration of boric acid in the liquid remaining in the vessel has a tendency to increase continuously, whereas the concentration in the reserve of cooling liquid which is recycled falls continuously.
It is then necessary to bring about an injection through the hot arms of the primary circuit in order to drive the cooling liquid having a high concentration of boric acid from the vessel and to cause the evaporation to cease. The direction of circulation in the vessel is then reversed and the cooling liquid injected can escape, after mixing, through the breach in the cold arm in liquid form.
In order to carry out this injection into the hot arms alternately with respect to the injection into the cold arms or simultaneously with this injection into the cold arms, there is provided in each of the main injection pipes of each of the trains a line tapped off towards the hot arms, which enters the enclosure and in which is located a block valve outside the enclosure.
These injection lines into the hot arms are generally grouped on drums depending upon the pressure of injection in the line in question. The drums as in the case of the cold arms ensure the distribution of the injected water into the various hot arms. These injection lines may equally well be grouped per train for injection into a hot arm.
In any case, if a rupture occurs at the level of a drum which groups the whole of the main pipes for injection into the cold arms at a certain pressure or the whole of the lines for injection into the hot arms at a certain pressure, the safety injection is no longer ensured for the range of pressure corresponding to this drum.
If the number of members providing the same function is multiplied in order to increase the safety of the device in operation, the complexity of the whole becomes too great.